1. Field of the Invention
The present invention relates to a differential amplifier and, particularly, to a differential amplifier that includes a differential pair in an input section.
2. Description of Related Art
In recent semiconductor devices, the implementation size of a complementary metal-oxide-semiconductor field effect transistor (CMOSFET) that is used as a circuit component, for example, has become miniaturized. With such miniaturization, a voltage applicable to a MOSFET is decreasing. On the other hand, a demand for higher performance and lower power consumption is increasing in portable electronic equipment into which a semiconductor device is incorporated.
One approach to meet the demand for lower power consumption is to set a lower power supply voltage. However, as a power supply voltage is set lower, the operation and design of a circuit becomes more difficult. For example, there is a limit to a valid input voltage range in an enhancement FET, both an Nch MOSFET and a Pch MOSFET, which is often used in an LSI. Specifically, in an enhancement FET, there is a threshold voltage which indicates a voltage level that cannot be used as a valid input voltage in order to keep an output current in the off state (which is called “normally-off”), and an Nch MOSFET does not operate unless an input voltage is equal to or higher than the threshold voltage, and a Pch MOSFET does not operate unless an input voltage is equal to or lower than the threshold voltage. However, if a power supply voltage is lowered, the proportion of a threshold voltage range, which is a non-active region for an element, to a power supply voltage range increases. Accordingly, the proportion of a voltage range that cannot be used as a valid input voltage range increases. Further, the voltage of a signal in the circuit becomes the threshold voltage or lower, causing a part of the circuit to become inactive.
To address such an issue, a method for realizing a circuit capable of dealing with reduction of a power supply voltage by means of circuit technology has been proposed. For example, a method of enlarging a valid input voltage range without consideration of the existence of a threshold voltage has been proposed in the MOSFET technology. Specifically, a circuit is proposed that includes both a differential amplifier circuit having an Nch MOSFET as an input portion and a differential amplifier circuit having a Pch MOSFET as an input portion, combines output signals of both circuits and thereby obtains a final output signal. One example of a differential amplifier in which a differential pair at an input portion is formed by transistors of different polarities is disclosed in Japanese Unexamined Patent Application Publication No. 6-237128.
FIG. 3 shows a circuit diagram of a differential amplifier 100 disclosed in Japanese Unexamined Patent Application Publication No. 6-237128. The differential amplifier 100 is an amplifier that uses bipolar transistors. The differential amplifier 100 includes a first differential input stage formed by NPN transistors Q1 and Q2, a second differential input stage formed by PNP transistors Q3 and Q4, a first output stage formed by a PNP transistor QC1 and an NPN transistor QC5, and a second output stage formed by a PNP transistor QC2 and an NPN transistor QC6. In the differential amplifier 100, signals input to the first and second differential input stages are amplified and output through the first and second output stages.
The differential amplifier 100 uses a common-mode voltage feedback circuit to let the first and second output stages operate. The common-mode voltage feedback circuit of the differential amplifier 100 is formed by PNP transistors QC3 and QC4, NPN transistors QC7, QC8, Q5 and Q6, and constant current sources ICMP and ICMN. The common-mode voltage feedback circuit detects a common-mode voltage difference of output voltages and supplies a common-mode current corresponding to the detected common-mode voltage difference to the first and second output stages.
Thus, the differential amplifier 100 incorporates the first differential input stage formed by NPN transistors and the second differential input stage formed by PNP transistors, thereby enlarging the input voltage range. Further, the differential amplifier 100 enlarges the output voltage range by the operation of the common-mode voltage feedback circuit.